1. Field of the Invention
The present invention relates to an industrial boiler for use in connection, for example, with an electric power plant.
2. Description of the Related Art
FIG. 9 illustrates a conventional steam boiler. In such a boiler, fuel and air are introduced into a furnace 51 from a burner 62 for combustion purposes. The combustion gas then passes superheaters 63 and 64 and a reheater 65, all provided above the furnace 51, and thereafter, is delivered to a rear flue 52. The rear flue 52 is divided by a partition wall 53 into a first gas passage 54 and a second gas passage 55. The first gas passage 54 contains a reheater 56 and an economizer 57. A damper 60 is arranged below the ecomonizer 57 to regulate the flow of gas in the first gas passage 54. The second gas passage 55 contains a superheater 58 and an economizer 59. A damper 61 is arranged below the economizer 59 to regulate the flow of gas in the second gas passage 55. With this arrangement, the temperature of steam is controlled by the regulation of the flow of gases passing through the first and second gas passages 54 and 55. In this type of boiler, however, the flow of gases is subject to change when a different type of coal is used, or when load output in the boiler is varied. If this occurs, the temperature of gas at the outlet of the first gas passage 54 is likely to differ from that of gas at the outlet of the second gas passage 55. Such gases are delivered to systems downstream of the outlets of the first and second gas passages 54 and 55 without mixing the gases to a full extent. This leads to malfunction of a denitrification system or an air heater, causing malfunction of the boiler. It should be mentioned that particularly in a dry denitrification system, excessive increase or decrease in the temperature of gases directed thereto may deteriorate the activity of catalyst. It should also be mentioned that conventional boilers provide no means for accommodating various types of coals.
Another conventional boiler is shown in FIG. 10, wherein like reference numerals designate like parts in FIG. 9. A dry denitrification system is provided at the downstream of the economizers 57 and 59. The temperature of gases at the outlet of the economizers 57 and 59 tends to decrease as load output in the boiler decreases. It is necessary to maintain the temperature of gases at the outlet of the denitrification system as high as possible for the maximum operating efficiency of the denitrification system as well as for the longer activity of catalyst. To this end, gas at upstream of the economizer 59 in the second gas passage 55 is partly directed to a by-pass passage. The gas in the by-pass passage then passes through a damper 67 and thereafter, is mixed with gases from the first and second gas passages 54 and 55. When the boiler steams at a low load output, the damper 67 is adjusted in such a manner to increase the flow of gas in the by-pass passage 66 and thereby to regulate the temperature of gases at the inlet of the denitrification system. In this boiler, however, no means is provided to equalize the temperature of gas at the outlet of the first gas passage 54 with that of gas at the outlet of the second gas passage 55. Thus, two different layers of gases flow in the duct and are delivered to the denitrification system. In order to maintain the minimum gas temperature, a large amount of gas is necessary in the by-pass passage 66. However, this may deteriorate the operating efficiency of the boiler while requiring the large by-pass passage 66.